TN0023 Technical note

Transcription

1 Technical note Discontinuous flyback transformer description and design parameters Introduction The following is a general description and basic design procedure for a discontinuous flyback transformer. It can be useful for further development of an existing design, where some parameters are known, or for communicating changes with a transformer design house. The flyback transformer can be used in isolated or non-isolated power supplies for almost any output voltage. The discontinuous flyback transformer has a triangular current waveform and commonly uses a 50% duty cycle at full load and minimum input line. A lower duty cycle can be used but the peak and RMS current is higher. A higher duty cycle involves more complicated means for stability and a lower bandwidth. The frequency is selected based on many considerations - for example, a smaller transformer can be used with higher frequency but the switching losses are higher. The application requirements dictate the design and include minimum AC input voltage, maximum output power (P Omax ), efficiency (η), operating frequency (f), output voltage (V o ) and diode drop (V do ). Some of the other circuit considerations, which are a result of the design, include switching device current rating, peak current (for peak current limit), switching device drain to source voltage and output rectifier reverse voltage (from maximum input voltage), wire size, and auxiliary voltage if required. This technical note presents one of the many procedures used to design a discontinuous flyback transformer for a low power application, including application requirements. August 2012 Doc ID Rev 2 1/7

2 Inputs TN Inputs P omax : the minimum DC voltage from the minimum AC input (V) : maximum output power (W) η: efficiency, initially based on experience f: operating frequency (Hz) V o : output voltage (V) V do : output voltage diode drop (V) V aux : auxiliary voltage (V) V da : auxiliary voltage diode drop (V) 1.1 Estimate Choose G in = in. A gap size range starting point is about to in. If the gap is too small the manufacturing tolerances are difficult and if the gap is too large, fringing around the gap can be a problem. Estimate the core size and use the effective core area (A e ). 1.2 Procedure 1. Determine the minimum DC input voltage from the minimum AC voltage minus about 20 V of ripple voltage. Choose a flyback voltage ( ) equal to the minimum DC input voltage. This sets the duty cycle (D). = V ACmin 2 20(V) = V min 2. Calculate the duty cycle, which is 50% or less for the discontinuous mode flyback. D max = Calculate the peak device current (I pk ). There may be a switching device limit (for example, the VIPer53-E has a current limit of 1.7 A). 4. Calculate the primary inductance (L p ). This is the inductance used for the current in the switching device to reach the peak current during the duty cycle. I pk 2P omax = η D max D max L P = f I pk 2/7 Doc ID Rev 2

3 Inputs 5. Calculate the primary turns (N P ): G N in 25.4 L P P = A e 4π10 8 where G in is in inches, and A e is in square centimeters. 6. Calculate the secondary turns (Ns) and auxiliary turns (N aux ): N s = N p (V o +V do ) / N aux = N p (V aux +V da ) / 7. Calculate the maximum flux density (Bm, Gauss), which depends on the core material. B m = L p I pk 10 8 / N p A e Flyback transformers vary in size and shape, but the most common is an E-E type core with a gap in the center leg. It is basically an inductor where the energy is stored in the center leg (note the polarity of the transformer windings). The cores have an effective core area (A e ) which can be estimated for a transformer design. A gap is selected and then other values are calculated. If the resulting parameters are not reasonable, another core can be selected with an A e which works better with the application. The bobbin area for the selected core is also difficult to estimate because it is the result of all previous calculations, wire size, number of turns and safety requirements such as creepage and clearance, and bobbin pin spacing. Core size starting points for the following devices are: VIPer22ADIP-E - 20 mm, and VIPer53-E - 25 mm. Reasonable results include D max 50%, I pk < a specified limit, a gap between and 0.03 in., B max < X000 Gauss (depends on core material), meets safety spacing requirements, does not run too hot and fits on the bobbin/core. 1.3 What if The wire does not fit: a) Choose a somewhat smaller gap which decreases the turns (check the flux density). b) Increase the operating frequency, which decreases the number of turns and the flux density. c) Reduce the flyback voltage, which increases the peak current (check the limit), reduce the primary inductance and reduce the number of turns (the flux density will be the same). d) Select a larger bobbin/core. The flux density is too high: a) The gap can be increased somewhat which also increases the number of primary turns (check the size). b) Increase the operating frequency, which decreases the number of turns and the flux density. c) A larger core with a larger effective core area (A e ) can be used. The peak current is too high: a) Use a device with a higher peak limit. b) Check that the duty cycle is up to 50%. Doc ID Rev 2 3/7

6 Revision history TN Revision history Table 1. Revision history Date Revision Changes 18-Jan First issue 03-Aug Updated Section 1.2: Procedure, list item 5 to specify the units of measure for G in and A e Changed all references to part number VIPer53DIP-E to VIPer53-E Removed references to VIPer12ADIP-E Minor text changes throughout the document 6/7 Doc ID Rev 2

7 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries ( ST ) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein. UNLESS OTHERWISE SET FORTH IN ST S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. UNLESS EXPRESSLY APPROVED IN WRITING BY TWO AUTHORIZED ST REPRESENTATIVES, ST PRODUCTS ARE NOT RECOMMENDED, AUTHORIZED OR WARRANTED FOR USE IN MILITARY, AIR CRAFT, SPACE, LIFE SAVING, OR LIFE SUSTAINING APPLICATIONS, NOR IN PRODUCTS OR SYSTEMS WHERE FAILURE OR MALFUNCTION MAY RESULT IN PERSONAL INJURY, DEATH, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE. ST PRODUCTS WHICH ARE NOT SPECIFIED AS "AUTOMOTIVE GRADE" MAY ONLY BE USED IN AUTOMOTIVE APPLICATIONS AT USER S OWN RISK. Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any liability of ST. ST and the ST logo are trademarks or registered trademarks of ST in various countries. Information in this document supersedes and replaces all information previously supplied. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Philippines - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America Doc ID Rev 2 7/7

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